When mobile user equipment (UE) is moving in a mobile communication system such as a long term evolution (LTE) system, it often travels through different cells. If the mobile UE is not using a communication service provided in the LTE system, the mobile UE will notify the network every now and then that it has moved to another cell. If the mobile UE is engaged in a specific communication service in the LTE system, the service needs to be maintained while the mobile equipment is moving to another cell and until the user stops the service. The process of replacing communication with one cellular radio station (e.g., evolved NodeB (eNB)) with another cellular radio station is referred to as handover.
Handover is one of the most important functions of the LTE system, which tries to keep the UE connected to the best eNB such that quality of service (QoS) of the ongoing session is met. In the LTE system, handover is usually based on the downlink and/or uplink channel measurements which are processed in the UE.
Generally, a source eNB initiates handover when the signal quality falls below a predetermined value by transmitting a handover request message to another eNB as a target eNB. Conventionally, when at least one evolved radio access bearer (E-RAB) is allocated in the target eNB, the target eNB sends a handover request acknowledgement message to the source eNB with partially allocated E-RAB information. In such a case, UE may experience service drop after handover. In the case of VoLTE service, which provides voice service by using two E-RABs (e.g., a media E-RAB and a signaling E-RAB), if the media E-RAB is not allocated at the target eNB and the signaling E-RAB is only handed over to the target eNB, UE may not be able to transmit and receive any voice packet to and from the target eNB.
Further, if at least one E-RAB is granted in the target eNB, resources of the granted E-RABs are allocated in the target eNB and the handover request acknowledgement message is sent to the source eNB with partially allocated E-RAB information. Thus, if the source eNB sends a handover cancellation message to abort handover to the target eNB based on the partially allocated E-RAB information, the target eNB releases the previously allocated E-RAB. In this case, the target eNB has to maintain the partially allocated E-RABs until the handover cancellation message is received from the source eNB. As a result, the resources at the target eNB may become unavailable temporarily so that another UE's connection setup request, an additional E-RAB setup request, or another incoming handover request may be restricted. In addition, unnecessary messages such as a handover cancellation message, etc. between the source eNB and the target eNB may increase signaling load on S1 or X2 interfaces.
Furthermore, if the target eNB sends a handover preparation failure message with cause to the source eNB, the handover will be stopped. In this case, UE may experience degraded quality of service because the UE remains in the source cell with poor radio environment until another neighbor cell, whose handover conditions are met, is found and handover is triggered again. Additionally, the handover preparation and cancellation may be repeated if the target eNB has fewer available resources during a busy period of use and a partial E-RAB setup occurs frequently.